Because that's the way science works. If it's not supported, then it's not a theory anymore, it's at best a disproved theory. ID/creationist supporters keep wanting to apply their own definitions to these things, it's a semantic battle. Scientists look at the data and find a way to explain it that makes sense. If the data doesn't support the explanation, they throw it away and think of a new one. So far in the 100-odd years that evolution has survived as a theory, all we get is more and better evidence to support it as the tools get better. ID supporters have an agenda to push and are ignoring any evidence that doesn't support it. They aren't going to throw away their "theory" on the basis of anything so silly as facts and evidence.